MEMS Seminar: Adsorption at Confined Interfaces
Wednesday, January 30, 2019 - 12:00pm to 1:00pm
Dr. William Ducker | Virginia Tech
Material at interfaces has different properties than bulk solutions; here we are concerned with the properties of materials that are in the thin film between two interfaces. The properties of such confined material are affected by fields extending from two boundaries and thus are different than material near one interface. These altered properties are important in determining the stability of colloid and nanoparticle suspensions, wetting films, adsorption in confined spaces, and in the fabrication and application of nanoscale devices. Our interest is in adsorption, which affects many of these applications: there is a multitude of applications where surfactants, polymers, ions, etc. are adsorbed to effect changes in thin films, for example, to alter the stability of colloidal particles.
We describe measurements of adsorption between two flat plates when the plates are separated by 0 – 65 nm for two examples (1) depletion of a simple ion in dilute solution and (2) adsorption in very concentrated salt solutions. These measurements have been made possible by our development of a technique to measure adsorption in a crack formed by bonding an oxidized-silicon wafer to a glass wafer. Measurement of film thickness is achieved by using optical interference. The adsorbed amount at each film thickness is measured from the fluorescence emission of a dye, after accounting for the optical interference.
One of the measurements is of the depletion of a divalent anion, fluorescein, in aqueous solution between two anionic solids. For dilute solutions at large separations between the flat plates, the dye is depleted relative to the bulk concentration. At smaller separations, the depletion of the dye decreases. The range of the depletion and the magnitude of depletion decrease with shorter Debye-length. Both of these effects are consistent with a simple calculation using the Poisson-Boltzmann equation. The other example is in concentrated solutions. Consistent with the work of others, our results do not agree with Poisson-Boltzmann theory, and our results show that the electrostatic screening length is many nanometers in highly concentrated (>2 M) solutions . Poisson Boltzmann theory predicts that the potential decays exponentially with a decay length (Debye-length) that decreases with increasing concentration. Results are consistent with an increase in decay length with increasing concentration. The failure of Debye-Hückel/ Poisson Boltzmann Theory in concentrated solutions is not unexpected, but the scale of failure is. We make comparisons to results in ionic liquids and drawn conclusions for crystal growth through particle attachment.
Lunch will be served at 11:30 am.
Hosted by Dr. Stefan Zauscher.